A mobile station (sometimes called User Equipment or “UE”), in a cellular network that is connected to a serving cell, normally does not read system information transmitted by a neighbor cell. There are several situations, however, where a UE having an ability to read a neighbor cell's system information might be useful. Some of these situations are described below.
(1) To enable handover to a Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) Home enhanced Node-B (HeNB): A serving cell may not be able to determine the correct target cell using only the physical cell identity (PCID) reported to the serving cell in a measurement report sent by the mobile station to the serving cell. In this case the mobile station may benefit from reading system information from neighbor cells to obtain an additional identifier (e.g. CSG ID, cell global ID) of the target cell.
(2) Self Optimized Networks: Networks are moving to a self organization/self optimization model. With increasingly complex networks (e.g., relays in LTE-advanced), mobile stations may be expected to read and report system information of neighbor cells.
(3) Drive test minimization: There is an ongoing effort in standards bodies such as 3GPP to define mobile station measurements and procedures to reduce the need for drive testing to optimize network parameters. This is likely to expect mobile stations to read system information of neighbor cells.
(4) Knowing a downlink bandwidth of a target cell may allow a UE to make more accurate measurements and improve handover performance. In LTE, the downlink bandwidth is transmitted in the Master Information Block, which is a format for system information.
(5) Knowing the System Frame Number (SFN) of a target cell, random access channel configuration information of the target cell, and other system information of the target cell before a handover would be beneficial in reducing handover interruption. SFN is transmitted in a Master Information Block (MIB) and the random access channel configuration information is transmitted in a System Information Block 2 (SIB2).
(6) In general, knowing neighbor cell system information can facilitate a quick recovery in case of a handover failure or a radio link failure.
If adequate idle periods are available in connected mode (DRX periods), a mobile station can autonomously read neighbor cell system information. However system information messages are repeated relatively infrequently (80 ms periodicity for SIB1, 160 ms for SIB2, and so on). The mobile station may need to receive multiple transmissions of a system information message before it can successfully decode the message. The mobile station may be unable to find such extended idle periods. Alternatively, the mobile station could be assigned a measurement gap to tune to the neighbor cell and obtain the neighbor cell's system information. During this assigned measurement gap, the serving cell would not schedule any transmissions for the mobile station. Such a measurement gap, however, would have to be very long because reading MIB and SIB1 alone—which is a pre-requisite for reading other SIBs—can require a measurement gap of up to 160 ms. Such long gaps cause significant interruption to the mobile station's connection to the serving cell.
Therefore, it would be beneficial to have other mechanisms to acquire the system information of neighboring cells while reducing interruptions to the connection between the mobile station and the serving cell.